Conventionally, such as in a color printer, color copier, and display device, a color signal is converted to a different color space or to a different color gamut; and specific colors are converted with hue, chroma, and lightness respectively. To perform various types of color management processes with a high degree of flexibility, as well as precisely and quickly, a color management process is performed on the basis of a three-dimensional look-up table (described as LUT hereinafter).
However, preparing a large number of LUTs for optimizing color management such as according to media, signal source, and mode of color conversion, respectively, requires an extremely large memory capacity. For this reason, a technology, together with simplified LUT and matrix calculations, for example, has been developed implementing a color conversion process according to these various types of modes.
For example, patent document 1 describes an example of such a technology. FIG. 14 shows a color conversion apparatus disclosed in patent document 1. In FIG. 14, the color conversion apparatus includes input device 9000, color conversion part 9001, and output apparatus 9007. Color conversion part 9001 includes three-dimensional LUT calculation part 9002, matrix calculation part 9003 with its matrix coefficient variable, and one-dimensional LUT calculation parts 9004, 9005, 9006. Three-dimensional LUT calculation part 9002 receives a signal from input device 9000 to process a signal received from input device 9000 using a three-dimensional LUT. Matrix calculation part 9003 matrix-calculates an output from three-dimensional LUT calculation part 9002. One-dimensional LUT calculation parts 9004, 9005, 9006 calculate an output from matrix calculation part 9003 using a one-dimensional LUT. The outputs from one-dimensional LUT calculation parts 9004, 9005, 9006 are fed to output apparatus 9007. Color conversion part 9001 uses plural matrix coefficients corresponding to various types of color conversion processes. As such, each color conversion process mode can be handled without changing a three-dimensional LUT. In this color conversion part 9001, three-dimensional LUT calculation part 9002 is not flexible, and thus regular changing of color conversion mode is handled by changing a matrix coefficient of matrix calculation part 9003 and the LUT content of one-dimensional LUT calculation parts 9004, 9005, 9006.
Meanwhile, when applying a color management system according to a three-dimensional LUT to a display unit, preparing plural LUTs according to a color conversion process mode and to a signal source requires extremely large memory capacity. Further, to rewrite data in a three-dimensional LUT requiring a large memory capacity needs a long rewriting time, which is unsuitable for a real-time process while displaying a moving image.
Under the circumstances, a color conversion apparatus in view of this problem has been devised. FIG. 15 shows such an example, which is a color conversion apparatus disclosed in patent document 2. In FIG. 15, the color conversion apparatus includes first image signal converter 9102, color corrector 9103, second image signal converter 9104, and two-dimensional LUT storage 9105. First image signal converter 9102 converts input image signal 9101 to each variable of hue, lightness, and chroma using given (i.e., predetermined) formulas. Color corrector 9103 color-corrects variables of hue, lightness, and chroma converted from an input image signal by first image signal converter 9102. Color corrector 9103, when color-correcting, uses a two-dimensional LUT produced by combining two variables out of hue, lightness, and chroma, preliminarily created and retained in two-dimensional LUT storage 9105. Second image signal converter 9104 generates output image signal 9106 by applying a given second expression to hue, lightness, and chroma after color-correction, which are output from color corrector 9103. Such a configuration does not decrease the operation speed, and thus is suitable for a real-time process.
However, color conversion apparatus 9001 shown in patent document 1 aims at conversion of limited chroma and lightness; and at low-cost correction according to media and a print mode. Further, the three-dimensional LUT is not flexible. Accordingly, with adjustment of a color conversion process by changing matrix coefficients alone, the conversion characteristic becomes linear, and flexibility in color conversion is restricted within a certain range. Consequently, a color conversion process with a high degree of accuracy, such as converting only a color with a specific hue, lightness, and chroma to a color with those different; and color gamut conversion with a nonlinear characteristic.
In the case of the color conversion apparatus and color conversion method shown in patent document 2, an LUT to be used is two-dimensional and is not updated appropriately although real-time process is possible. Consequently, color conversion process with a high degree of accuracy cannot be performed.    [Patent document 1] Japanese Patent Unexamined Publication No. 2001-203903    [Patent document 2] Japanese Patent Unexamined Publication No. 2007-19970